1. Field of the Invention
The present invention concerns a method to generate an optimized MR image of an examination subject, a corresponding electronically readable data storage medium, and a corresponding magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance (MR) imaging is a known modality with which images of the inside of an examination subject can be generated. Expressed in simple terms, for this purpose the examination subject is positioned in a strong, static, homogeneous basic magnetic field B0 (field strengths from 0.2 to 7 Tesla or more) in a magnetic resonance apparatus so that nuclear spins in the subject orient along the basic magnetic field. For spatial coding of the measurement data, rapidly switched gradient fields are superimposed on the basic magnetic field. To trigger nuclear magnetic resonances, radio-frequency excitation pulses (RF pulses) are radiated into the examination subject by at least one transmission coil. The triggered nuclear magnetic resonances (signals) are measured by reception coils, and MR images, for example, are reconstructed on the basis of the measured signals. The magnetic flux density of these RF pulses is typically designated with B1. The pulse-shaped radio-frequency field is therefore generally also called a B1 field for short. The nuclear spins of the atoms in the examination subject are excited by these radio-frequency pulses such that they are deflected (flipped) out of their steady state parallel to be basic magnetic field B0 by an amount known as an “excitation flip angle” (also called a “flip angle” in the following for short). The nuclear spins then precess around the direction of the basic magnetic field B0. The magnetic resonance signals that are thereby generated are acquired by radio-frequency reception antennas. The acquired measurement data are digitized and stored as complex numerical values—raw data—in a k-space matrix. By means of a multidimensional Fourier transformation, an associated MR image can be reconstructed from the k-space matrix populated by such values. In addition to anatomical images, spectroscopic data, movement (flow) data or temperature data of an examined or treated area can be determined using suitable magnetic resonance techniques.
The measured signals thus depend on the radiated RF pulses. In addition to a homogeneous basic magnetic field and precisely linear gradient magnetic fields for spatial coding, typical methods to reconstruct image data sets from magnetic resonance signals also require a homogeneous RF field distribution (B1 field distribution) in the examination volume. However, the B1 field distribution in the examination volume typically is non-uniform in real MR systems, which leads to image inhomogeneities (image artifacts in the MR images reconstructed from the signals, and therefore to a poorer ability to recognize the desired details in the imaged examination subject. Particularly in whole-body imaging or acquisitions of the torso (breast, abdomen, pelvis) at basic magnetic fields of 3 Tesla or more, artificial shadows occur in the image due to an inhomogeneous RF field distribution. Due to the resulting poor image quality, this has prevented more extensive use of such examinations in the clinical field. The interfering image artifacts intensify and multiply with an increase of the field strengths that are used.
Artifacts and inconsistencies in MR imaging or spectroscopy due to inhomogeneous B1 fields have long been known. In conventional MR imaging with a transmission channel, it is not possible to directly affect the homogeneity of B1 fields. Therefore, conventional methods have the goal of exciting signals over a defined range of B1 variations, which signals are independent of the B1 field strength. One example of such a procedure is the use of composite RF pulses or adiabatic RF pulses. However, these have only limited applicability with regard to achievable flip angle and phase response in use with slice selection, as well as with regard to pulse times and SAR intensity (SAR: “specific absorption rate”). They are therefore typically used only for magnetization preparation and moreover have found no broad application in typical MR imaging sequences. Such magnetization preparations can in turn reduce the sensitivity of a subsequent imaging sequence, but they naturally lengthen the required transmission time of the complete pulse sequence.
A further known procedure attempts to achieve a spatial modulation of the generated transversal magnetization by simultaneous action of RF and gradient pulses on the spin system. The achievable homogeneity of these spatially selective, two-dimensional (2D) or three-dimensional (3D) pulses is unlimited in principle. 2D and 3D modulations, however, lead to very long pulse times and an inefficient use of the RF pulses since the average flip angle per radiated power is reduced. Most notably, the length of the required pulse sequences has previously prevented an establishment of this method in MR imaging.
Using the parallel transmission technique—thus using multiple transmission/reception coils—it is possible directly influence the spatial distribution of the B1 field. The individual RF fields thereby emanating in parallel from spatially separated individual transmission coils are vectorially superimposed in order to form the actual B1 field. The generated B1 field can be spatially modulated by adjusting the phases and amplitudes of the individual transmission channels (transmission coils). This method is known as “RF shimming”. The achievable homogeneity is limited by the hardware, i.e. the number of available parallel transmission channels, for example. The pulse times for the spatially selective 2D and 3D pulses that are described above can also be shortened with the parallel transmission technique. However, the achievable pulse times are still longer than could be used to replace the previously typical slice-selective or non-selective pulses in the prevalent imaging sequences. Furthermore, it is unclear how robustly (i.e. with regard to B0 field variations, chemical shifts, movements of the examination subject and/or relaxation times), and with what SAR efficiency, these complex pulses can be used for in vivo imaging.
DE 103 38 074 B4 (corresponding to United States Patent Application Publication No. 2005/0083054) describes a method to compensate for contrast inhomogeneities in magnetic resonance images that are caused by spatial distributions of a transmitted radio-frequency field, in which method multiple individual images of a defined region are initially acquired with different radio-frequency pulse sequences which lead to different flip angles, and then on the basis of the various individual images a common contrast-homogenized image for the appertaining region is generated so that, within the contrast-homogenized image, intensity fluctuations caused by a distribution of the flip angle are at least smaller per region than in the individual images.